This invention relates to an improved motor control means and more particularly, to an improved motor control means for a two-speed single-phase refrigerant compressor motor.
Conventional single-phase alternating current induction motors for driving refrigerant compressors are connected to a source of power by a line contactor. In circuit with the main motor winding and the start winding are a run capacitor and a start capacitor. A start relay including a coil in parallel with the motor start winding or windings and normally closed start relay contacts in series with the start capacitor are provided. When the contactor is energized, its contacts close. The start relay coil senses the start winding voltage and is energized at a predetermined value as the start winding voltage increases during motor acceleration. Upon attainment of the predetermined value and energization of the start relay coil, the start relay contacts open. The start capacitor is operatively removed from the circuit. Assuming that the start relay is properly sized for the application, it will stay energized during all expected load and voltage conditions.
When the line contactor is deenergized and power is removed from the motor circuit, the main winding and start winding voltages drop to zero. The start relay coil is deenergized and the start relay contacts close. If the start relay coil should happen to open when the run capacitor is fully charged, there is a large amount of energy stored in the run capacitor that can be discharged into the start capacitor when the start relay contacts close. There is sufficient energy to weld the start relay contacts and thus disable the relay for the next operation.
An improved motor control for a single phase two-speed motor for a refrigerant compressor is disclosed in Pfarrer et al, U.S. Pat. No. 4,066,937, the teachings of which are incorporated herein by reference. In Pfrarrer et al, the motor control incorporates a negative temperature coefficient thermistor for controlling the discharge of the run capacitor so as to prevent welding of the start relay contacts. However, problems with the Pfarrer et al motor control have arisen due to limitations on the size of the start capacitor, which prevent the use of better starting torques.
In Pfarrer, U.S. Pat. No. 4,082,989, there is disclosed a two-speed motor control incorporating a start relay that works on both 2-pole and 4-pole connections. However, due to the different starting characteristics of the two start windings in the two-speed motor, it is very difficult to size a start relay that operates effectively on both 2-pole and 4-pole connections. From this background, the present invention was developed.